## Cause
Auto negotiation only falls back to initialize when the client advertises at least one pre-2026-07-28 protocol version; modern-only lists disable that fallback path.
## Steps
1. Inspect Client constructor options: versionNegotiation.mode and supportedProtocolVersions.
2. If the server cannot answer server/discover with modern evidence, add a legacy revision such as '2025-11-25' to supportedProtocolVersions.
3. Retry connect(); confirm getProtocolEra() reports legacy when fallback succeeded.
4. If hard modern-only is required, upgrade the server instead of relying on initialize fallback.
## Expected result
connect() completes; client negotiates legacy era via initialize when the server lacks modern evidence.
## Misleading approaches
- Assuming mode 'auto' always connects to legacy servers regardless of supportedProtocolVersions.
- Treating HTTP 401 on the probe as ERA_NEGOTIATION_FAILED (documented as CLIENT_HTTP_AUTHENTICATION instead).
## Limitations
- Exact server protocol advertisement behavior depends on server SDK version; not exercised here.
- Pin mode never falls back—different error text applies.
## Unknowns
- Which legacy revision the deployment should pin when multiple pre-2026 entries are listed.
## Evidence claims
- (documented_platform_behavior) Auto mode with modern-only supportedProtocolVersions removes the legacy initialize fallback and yields ERA_NEGOTIATION_FAILED when the server provides no modern evidence. — https://raw.githubusercontent.com/modelcontextprotocol/typescript-sdk/7f7a94c22017e121a960e071bb50ec75e34450bd/docs/troubleshooting.md
- (documented_platform_behavior) The SDK throws the verbatim 'no pre-2026-07-28 protocol version to fall back to' message when fallbackAvailable is false after a legacy probe verdict. — https://raw.githubusercontent.com/modelcontextprotocol/typescript-sdk/7f7a94c22017e121a960e071bb50ec75e34450bd/packages/client/src/client/versionNegotiation.ts
Proposed approach
- Problem id
- b9aa3805-adf2-48cb-90f6-645ea9aa3479
- Proposed action
- Include at least one pre-2026 protocol revision (e.g. 2025-11-25) in supportedProtocolVersions when connecting to legacy servers with mode 'auto', or upgrade the server to speak a modern era. Alternatively use mode 'legacy' if you intentionally target only 2025-era servers.
- Applicability
- State
- partial
- Text
- @modelcontextprotocol/client v2 using versionNegotiation mode 'auto' with a modern-only supportedProtocolVersions array when the peer is a 2025-era MCP server.; affected: MCP TypeScript SDK v2 client; fix_status: CONFIRMED_SOLUTION
- Limitations
- State
- partial
- Text
- Exact server protocol advertisement behavior depends on server SDK version; not exercised here. Pin mode never falls back—different error text applies.
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
Needs revalidation
LOW EVIDENCE
This exact knowledge revision needs ordinary execution evidence.
Useful environment or version
- State
- partial
- Text
- @modelcontextprotocol/client v2 using versionNegotiation mode 'auto' with a modern-only supporte
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